Biology Revision: Cell Activity


Cell Structure

Animal cells contain:

  • Nucleus-Controls the cell's activities
  • Cytoplasm-Most of the chemical reactions take place here
  • Cell Membrane-Controls the passage of substances in/out of the cell
  • Mitochondria-Where energy is released through respiration
  • Ribosomes-Protein Synthesis

As well as this plant cells have chloroplasts (absorb light energy to make food) and a permanent vacuole filled with cell sap. Plant and algal cells also have a cell wall made of cellulose to strengthen the cell.

Bacterial cells contain only cytoplasm and a membrane surrounded by a cell wall: The genes are not in a nucleus, but in circular structures called plasmids.

Yeast is a single-celled organism with a nucleus,cytoplasm and a membrance surrounded by a cell wall.

There will also be cells which adapt from these structures in order to do specialised functions.

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The movement of substances in and out of cells

Diffusion-Movement from HIGH to LOW concentration. Examples include dissolved substances and 

Osmosis-Diffusion of water from a HIGH to LOW concentration across a SEMI-PERMEABLE membrane. Osmosis is caused by differences in the concentrations of solutions in and out of the cell.

Active Transport- almost the opposite of osmosis as it goes AGAINST the concentration gradient by using energy from respiration.

Single-celled organisms have a large SA:Volume ratio, and so they will use the cell membrane.

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Cell Division Part 1

The nucleus of a cell contains pairs of chromosomes, carrying genes. These genes have different forms known as alleles, which will produce different characteristics.

Mitosis- When copies of the genetic material are made, and then the cell divides to form two genetically identical cells. This is used by body cells to replace damaged cells or grow.

Meiosis- When the genetic material is copied and the cell divides TWICE to form four cells, each with a single set of chromosomes. This is because meiosis is used by gametes 

The gametes join at fertilisation to form a single body cell with new pairs of chromosomes. This cell will repeatedly divide by mitosis to form many cells, which will differentiate as the organism develops.

Animal cells will usually begin to differentiate at around the 30 cell mark, whilst plants can differentiate at any time in their life. Cell division in animals usually only applies to repair and replacement.

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Cell Division Part 2

Cells from human embryos and bone marrow, known as stem cells, can be manipulated to differentiate into various types of human cell, such as nerve cells.

In therapeutic cloning, the embryo is produced with the same genes as the patient, so that stem cells can be extracted from it which will not be rejected from the body.

This treatment could help against such problems as paralysis

Tumours will form from abnormal, uncontrolled growth of cells.

There are two types: benign, which do not invade other tissues, and malignant, which invade healthy tissue.

They can be caused by carcinogens from tobacco and asbestos, and ionising radiation such as UV and x-rays.

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